To read data from a file, the application first calls an operating system function

In file I/O, to read data from a file linux file handle, the application must first call the operating system function and pass the file name, and select a path to the file to open document. This function gets back a sequence number, that is, Perl file handle (filehandle) linux file handle . This Perl file handle is the only basis for identification of the open file. To read a piece of data from a file, the application needs to call the function ReadFile and transmit the address of the Perl file handle in video memory and the number of bytes to be copied to the operating system. When the task is completed, the file is closed by calling a system function.

Not only do you imitate a solipsist philosopher and write an artificial intelligence program, your program uses a method that does not communicate with the outside world. In the third and fourth lines of the class example, you will see "GRADES", which is a data type that refers to another Perl file, called a filehandle. A handle is a name you give a file, device, socket, or pipeline to help you remember the name you're working with and to hide the complexities of individual caches, etc. (Internally, handles are like streams in C, or I/O channels in BASIC.) Handles make it easy for you to input from and output to different places. One of the things that makes Perl a good language is its ability to communicate with multiple files and process them all at once. Having good symbolic names for external objects is an integral part of a good language [1].

Other things that make Perl a good language are: it is 8-bit, it is embeddable, and you can embed other programs in Perl through extension mode. It is concise and easy to use on the web. The environment is clear and easy to talk to. You can reference it in many different ways (as seen above). Actually, the language itself is not so strictly structured that you can't make it beyond your question. Back to TMTOWTDI again.

You create a handle and connect it to a file through the open function. open takes two parameters: the handle and the name of a file you want to link to it. Perl also provides some predefined (and pre-opened) handles. STDIN is the normal input channel of your program, and STDOUT is the normal output channel of your program. STDERR is an additional output channel so that the program can give some instructions when converting input to output [2].

Normally, these handles are linked to your terminal, so you can type your program and see it, but they can also be linked to files. Perl can give you this predefined handle because your operating system already provides it. Under UNIX, a process inherits standard input, output, and errors from its parent process (usually a shell). One of the responsibilities of a shell is to structure this I/O stream so that child processes don't have to worry about that).

Since you can use the open function to create handles for various purposes (input, output, pipeline), you must be able to specify what you want to do. Just like on the UNIX command line, you simply add characters to the file name.

Copy the code The code is as follows:

open(SESAME,"filename");#Read from an existing file

open(SESAME,"open(SESAME,">filename");#Create a file and write to it

open(SESAME,">>filename");#Continue writing to the existing file

open(SESAME,"|output-pipe-command");#Build an output filter

open(SESAME,"input-pipe-command|");#Build an input filter

As you heard, you can choose any name you want. Once a SESAME handle is opened, it can be used to access files or pipes until it is explicitly closed (with close(SESAME)), or a series of opens to the same handle links this handle to another file [3 ].

Opening an already open handle implicitly closes the first file, making it unavailable for Perl file handles, and opens a different file. You have to be mindful that this is what you really want to do. Sometimes, it happens that linux download tools happen by chance. For example, when you open($handle,$file), $handle happens to contain an empty string (null). Make sure to set $handle to a single value, otherwise you will open a new file with a null handle.

Once you have opened a handle for input (or you use STDIN), you can use the "line read operation" to read a line. This one is also known as a masonry operation, due to its shape. This masonry operation contains the handle()[4] you want to read. Use the STDID handle to read the answer provided by the user, as follows:

Empty masonry operation will read from all files specified on the command line. If not specified, read from STDIN. (This is standard behavior of many UNIX "filter" programs)

Copy the code The code is as follows:

printSTDOUT"Enteranumber:";#Request to enter a number

$number=;#Enter a number

printSTDOUT"Thenumberis$number";#Output this number

Do you understand the example we gave you? What does STDOUT do in the print sentence? This is one of the ways you use an output handler. A handle can be used as the first parameter of the print statement. If it exists, it tells where to output. In the example, the handle is redundant because the output is already STDOUT. The default for input is STDIN and for output is STDOUT. (We have omitted line 18 of the class counterexample to avoid confusing you.)

We also have one thing that you don't understand. If you try the example inside, you can notice that you get a very empty line. Because you did not manually remove the newline character from your input line when reading (for example, you entered "9"). For this case, when you want to remove newlines, Perl provides the chop and chomp functions. chop will indiscriminately delete (and return) the last character passed to it, while chomp will only delete the end of the record identifier (usually ""), and return the number of characters so deleted.

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